My exh left me with two children, they were 2 yrs ( autistic) and 3 months. He just disappeared. Gone! I had to fight for divorce, child maintenance etc.
He went on to have another child, mum of said child contacted me to say how awful I was keeping the kids away etc, he did the same to her.
He has just recently had another baby, with another woman, who has again contacted me to say how awful I am!
The kids are now 15 and 13! He has never asked for contact, owes an obscene amount of child maintenance. The 15 Yr old did try to contact him, at first it was lovely, then he turned and went on about how awful I was, how I took him for everything etc. My ds told him where to go!!
Mum of first child (after mine) is now in regular contact, the kids have met. We are not friends but we make an effort. We have joked that the group chat is open to the new one when she's ready!
I don't know why they do it, but if you do end up like this, be there for your kids, do your best to minimise the impact.